Investigation of unsteady flow conditions at dam bottom outlet works due to air entrainment during gate closure: Volume II

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The Berg River Dam is equipped with the first multi level draw off environmental flood release outlet in South Africa and can release flows of up to 200 m3/s. The outlet is controlled by a radial gate and is protected by a vertical emergency gate. Commissioning tests of the emergency gate in 2008 found that large volumes of air were expelled from the air supply shaft designed to reduce expected negative pressures in the conduit during emergency gate closure.
In 2009 the WRC commissioned Stellenbosch University to investigate the air entrainment phenomenon at dam bottom outlet works during gate closure. The study, comprising of tests on a 1:40 scale physical model and a two-dimensional numerical computational fluid dynamics (CFD) analysis, was inconclusive on the cause of the large air releases. Volume I of this report covers subsequent study using a 1:14.066 scale physical model. The accompanying Volume II covers a study using three-dimensional CFD analyses.
The investigation of the air entrainment phenomenon at dam bottom outlet works of the Berg River Dam consisted of two interrelated components. The first component comprised of tests on a 1:40 scale physical model and a two-dimensional numerical computational fluid dynamics (CFD) analysis, was inconclusive on the cause of the large air releases. Subsequently, a 1:14.066 scale physical model was used, and the results are covered in Volume I of the report. The second component consisted of a study using three-dimensional CFD analyses, and the results are reported in Volume II.
Simulations of continuous gate closure on the as-built physical model of the Berg River Dam outlet showed predominant inflow of air into the airshaft during emergency gate closure with short high speed air releases while the emergency gate was between 35% and 25% open. The problem was determined to be one of air blowback in the air shaft rather than continuous air release. The cause of the blowback was found to be the constriction of flow at the radial gate chamber. A number of modified model configurations were tested and recommendations were made for future design. The most crucial of these is that flow in high headed outlets should not be constricted.